If you can access the space; leave a light on and a radio for several days. If there are not eggs in the nest, remove the nest each time they build it. (If there are eggs, you have a dilemma. If you remove it with the eggs, they will lay more eggs once they rebuild elsewhere; but it is up to you whether or not you could do that.)
If you cannot access the space, I am not sure. You could try mothballs--the scent may repel them.
Several realistic rubber snakes nearby would be a real deterrent.
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